The invention relates to the production of asphaltic crude oil. More particularly, it relates to a method of producing an asphaltic crude oil from a subterranean reservoir formation while preventing plugging of the reservoir formation due to in-situ precipitation of asphalt.
Crude oil is able to hold asphalt in solution. The amount of asphalt a crude oil can dissolve depends on its composition, temperature, and pressure.
Formation plugging due to in-situ precipitation of asphalt is a problem of producing asphaltic crude with a near-saturation asphalt content. The asphalt comes out of solution when the pressure of the reservoir fluid drops below the asphalt precipitation or asphalt saturation pressure. Such a drop in pressure occurs when the oil is produced in a conventional, vertical well. Due to the inherent, inevitably high pressure draw-downs required to produce at commercial rates, the reservoir pressure in the proximity of the wellbore easily drops below the asphalt saturation pressure, creating conditions favorable for in-situ precipitation of asphalt.
Furthermore, the fluid pressure is further reduced while passing through the geobaric gradient on the way to the surface. Provided the wellbore pressure remains above the bubble point pressure, further precipitation and subsequent deposition in the well tubulars takes place. However, if the wellbore pressure drops below the bubble point pressure, no further precipitation of asphalt within the wellbore takes place.
Preventive and remedial methods have been developed and routinely used in field operations to cope with the problem of asphalt deposition in well tubulars. However, no practical, effective methods exist which prevent or remove asphalt deposits formed in the reservoir.